But all too often they also offer something else: an elevated risk of contracting rashes and diarrhea because failing urban pipes dump untreated sewage and polluted stormwater flows right into our beaches after heavy rains.

The number of these events is startling. Every year, NRDC releases Testing the Waters: A Guide to Water Quality at Vacation Beaches. In this year's report, we discovered that there were more than 20,000 days of closings and advisories in 2008 because beachwater exceeded public health standards.

For instance, one of Huffington's featured beaches, Zuma Beach in Los Angeles County, exceeded health standards for beachwater quality (indicating the presence of human or animal waste) 11 percent of the time in 2008. While some beaches fared worse, that's 4 percent higher than the national average, and shows that Zuma is prone to dirty runoff.

This is the case in communities all around our country, but the effect is more intense in metropolitan areas. When it rains on city streets, water rushes into storm drains pulling oil, toxins, pet waste, fertilizers, and trash along with it.

In many cities, stormwater gets passed through the same pipes as sewage, and when the system gets swamped by a downpour, the sewage sometimes bypasses the treatment plant and gets dumped raw--with all its cargo of infectious bacteria, viruses, and parasites-upstream from urban beaches.

This is what occasionally happens at San Francisco's Ocean Beach, another beach included in Huffington's contest and one popular with surfers. City officials closed certain stretches for several days in 2008 because mixed stormwater and sewage was discharged after heavy rains.

The best way to keep beaches open is to keep the pollution out of the water in the first place.

Federal and local governments can make this a priority by requiring better controls on stormwater and sewage. A key solution is to use something called low impact development--techniques that retain and filter rainwater where it falls, letting it soak back into the ground rather than running off into waterways.

But in the meantime, if you want to enjoy the last days of summer by taking a dip in your city's best swimming holes--and you want to avoid a trip to the emergency room--take a look at NRDC's beach guide before you go. It will tell you how well your city officials monitor local beaches and how often they have exceeded health standards.

You can also take a look at our 5-star rating guide for 200 of the nation's most popular beaches. This is a good place to find swimming options that are fun and healthy. The water at these beaches is monitored more than once a week and almost always meets public health standards. In addition, the public is promptly informed whenever contamination is found.

Here is a sample of some well rated beaches (5 stars):

Gulf Shores Public Beach (AL)

Laguna Beach-Main Beach (CA)

Bolsa Chica State Beach in Huntington Beach (CA),

Newport Beach (CA)

Ocean City (MD)

Park Point - Community Club Beach in Duluth (MN)

Hampton Beach State Park in Hampton (NH).

And for tips on how to have a healthy trip to the beach, check out this live chat with me from earlier in the summer.

About

Switchboard is the staff blog of the Natural Resources Defense Council, the nation’s most effective environmental group. For more about our work, including in-depth policy documents, action alerts and ways you can contribute, visit NRDC.org.